Natural variation present in Arabidopsis thaliana will be used to identify new components of the light signal transduction cascade. Arabidopsis is an ideal candidate for these studies because it is found in many different geographical locations and must adjust to different light environments. Specifically, two RILs (Recombinant Inbred Lines) will be genotyped and a QTL (Quantitative Trait Loci) map created for each of the two populations under 5 different light conditions: red, far-red, blue, white, and dark. One QTL identified in the above experiments will be cloned to identify the gene responsible for the altered phenotype. Using molecular biology and biochemical techniques the gene and gene products will be characterized. Hopefully this approach will lead to the identification of a new light-signaling component, bringing us closer to understanding a complete signal transduction pathway. The ability to use natural variation to identify functional differences of genes has implications for the diagnosis of human disease. The methods described in this proposal can be used to determine human predisposition to heritable disease as well as susceptibility to environmental factors. Given the importance of quantitative genetic variation in health and disease, understanding the molecular basis of QTLs will advance our understanding of complex genetic traits.